The structurally bound, adenine nucleotide transforming enzymes of the myofibril will be studied as functional multienzyme complex with the thick filaments. The criteria to be experimentally established include constancy of composition, specificity of binding sites, functional unity in providing energy for the muscle cell, and possible reciprocity in enhancing the ability of the enzymes to perform their physiological role. The cardiac muscle in which a high proportion of the energy transducing enzymes are known to be compartmentalized provides an ideal system to investigate the complex. Hypoxic heart, which show alterations in nucleotides involved in energy transduction and show diminished contractility will be investigated for possible impairment in the myofibril-enzyme complex.